Human beings are capable of a wide variety of gas flow rates of from, for example, 0.1 liter per second to 16 liters per second. Moreover, many medical studies require that small changes in lung function be measured accurately and reproducible over time.
Another characteristic of gas flow which has medical significance is the rate at which gas flow changes. Flow that changes rapidly is described as having a high frequency constant. If a medical ventilator is attached to a patient, it may superimpose relatively high frequencies during inspiration. The ability of a gasflow measuring device to measure high frequency changes in flow rate is described as the "frequency response" of the device.
A Rotometer is another type of flow meter designed to measure steady, unchanging Oxygen flow. But these devices have too much inertia to respond quickly to rapid change in flow. Rotometers, thus have a low frequency response and would not make good Pneumotachs, for example, for diagnostic spirometry. Rotometers would respond inadequately at high flow rates and report lower than actual peak rates. Any Pneumotachs with high inertia do not respond quickly enough to rapidly changing flow rates. Accordingly, Pneumotachs may overstate or understate the airflow rates during, for example, a forced vital capacity (FVC) maneuver.
Another requirement for accurate airflow measurement is that the airflow measuring device should not impede the patients breathing ability. Consequently, the device should not have a high resistance to airflow. A reasonable maximum back pressure which a patient should encounter with such a device is 1.5 cm water per liter per second air flow.
Other requirements for flow rate measuring devices are that dead space volume of the device should be less than 5.0 milliliters for nonatal monitoring and less than 15 milliliters for adult monitoring.
One major problem with the use of Pneumotachs for ventilation monitoring is moisture and patient secretions which may render the device inoperative.
Prior art Pneumotachs fail to meet these various requirements. A typical Pneumotach is characterized by a bundle of capillary tubes which provide a small fixed resistance to gas flow. Flush openings (air taps) at both ends of the capillary tubes are used to measure the pressure difference created when gas flows through the device. The resulting pressure difference is very low, usually less than two centimeters of water. An electronic pressure transducer is used to measure the rapid changes in pressure when gasflow changes rapidly.
The capillary tubes usually are made of brass and the case is made of stainless steel. Different sizes are used for the capillaries to cover different flow ranges. Some Pneumotachs include capillaries formed in extruded ceramic material. The ceramic tends to absorb moisture thus avoiding tube occlusion to some extent. But such devices are large usually requiring sufficient length of tubing to produce laminar flow. This reduces portability and increases dead space.
Other types of Pneumotachs utilize fine mesh screens rather than capillaries. The screen Pneumotachs exhibit decreased dead space, slightly better frequency response, and easier disassembly for cleaning. Pneumotachs with three screens are also available. The center screen acts as a resistance element; the other two act to smooth the flow of gas. The screens also may be heated to reduce water condensation without heating the gasflow through them.
But screens clog easily and need frequent cleaning. They have been protected by filters which are disposable. But, the use of filters is limited by the fact that filters increase resistance and expense.
Large orifice Pneumotachs have been developed to permit passage of water droplets. But turbulent gasflow results requiring more sophisticated electronics to linearize the output. "Variable Orifice" Pneumotachs were developed to linearize the output mechanically.
Hot wire and turbine Pneumotachs have been developed for measuring airflow by its cooling properties and by the rate of rotation of turbine blades, respectively. The first of these have not proven to be very accurate for gasflow measurement because they become affected by moisture and non laminar flow. The second is affected by fluid composition, is unidirectional, and is limited in its frequency response by its inertia and momentum.
Similarly, Vortex Pneumotachs are inaccurate at low flow rates because they require fully developed turbulent flow which is achieved only above a certain flow velocity. Also, a long upstream straight section is required to prevent uneven flow patterns. Ultrasonic Pneumotachs have been developed to count vortices generated in the airflow and the speed of ultrasonic waves travelling through the tube. But they are expensive and very sensitive to change in density and temperature.
Although considerable development effort has gone into Pneumotach design, no ideal Pneumotach has become available. Those that are available are limited in range, are expensive, need extensive care and frequent calibration.